Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $217.26   10.354%   $239.7551   $239.76   $239.75   $239.80  
  Purchase #2 $142.84   11.786%   $159.6751   $159.68   $159.70   $159.70  
  Purchase #3 $34.64   13.520%   $39.3233   $39.32   $39.30   $39.30  
  Purchase #4 $96.77   12.564%   $108.9282   $108.93   $108.95   $108.90  
  Purchase #5 $195.63   5.469%   $206.329   $206.33   $206.35   $206.30  
  Purchase #6 $81.75   12.807%   $92.2197   $92.22   $92.20   $92.20  
  Purchase #7 $146.28   4.715%   $153.1771   $153.18   $153.20   $153.20  
  7 purchase totals = $915.17       $999.4075   $999.42   $999.45   $999.40